The present invention relates to catalysts containing carbon nanotubes, methods of making catalysts containing carbon nanotubes on porous substrates, systems employing carbon-nanotube-containing catalysts, and reactions catalyzed in porous carbon nanotube-containing catalysts.
Catalysts are crucially important in controlling chemical reactions in virtually all aspects of our lives. For example, catalysts are used to lower the temperature, increase the rate, and control the products in chemical reactions. While catalysts can be liquids or gases, solid catalysts are especially attractive for commercial applications because they are easy to store and transport, are readily separated from product streams, tend to be more environmentally benign, and can provide superior performance and greater control of a reaction.
A well-known problem with solid catalysts is slow heat and/or mass transfer. That is, with solid catalysts and systems employing solid catalysts, the speed of a chemical reaction can be limited by the time necessary for heat to travel to or from the catalyst or by the time needed for chemicals to get to and from the catalyst.
For many years, scientists and engineers have sought better catalyst materials with improved heat and/or mass transport properties. While there are probably thousands of publications and patents that address these problems, two recent patents are discussed herein.
In one approach, van Wingerden et al., in U.S. Pat. No. 6,099,965, described methods of making catalysts having desirable heat transport properties. In one example, particles of an iron-chromium alloy are placed in a steel pipe and sintered in a hydrogen atmosphere. The sintered particles are then heat treated in air and then treated with a suspension of alumina and Fe2O3/Cr2O3.
Tennet et al., in U.S. Pat. No. 6,099,965, described a catalyst comprising a rigid carbon nanotube structure and a catalytically effective amount of a catalyst supported thereon. Numerous advantages of this structure including enhanced heat and mass transfer are dicussed (see col. 16, lines 8-65).
Despite the prior work, there remains a need for novel solid catalyst materials that have superior heat and/or mass transport capabilities.
In a first aspect, the invention provides an engineered catalyst that includes a support material having through-porosity (defined as discussed below), a layer comprising carbon nanotubes on the support material; and a surface-exposed catalytically-active composition.
In another aspect, the invention provides catalyst including a support; nanotubes dispersed over the support; and a catalytically-active composition dispersed over the nanotubes.
In yet another aspect, the invention provides a method of forming a porous carbon nanotube containing catalyst structure. In this method, a large pore support is provided having through porosity. Carbon nanotubes are formed over the large pore support, and a catalyst composition is deposited over the carbon nanotubes.
The invention also includes methods of conducting catalyzing chemical reactions in which one or more reactants are contacted with any of the carbon nanotube containing catalysts described herein. In this method, the one or more reactants react to form a product. The catalyst catalyzes the reaction relative the same reaction conducted in the absence of a catalyst. For example, the invention provides a Fischer-Tropsch process in which a gaseous composition, comprising CO and hydrogen, is passed over any of the carbon nanotube containing catalysts described herein.
The invention also provides a catalytic process for aqueous phase hydrogenations to produce higher value chemical products from biomass feedstock.
In another aspect, the invention provides a process of making a porous, carbon nanotube-containing structure, comprising: providing a support material having through-porosity; depositing seed particles on the support material to form a seeded support material; and heating the support material and simultaneously exposing the seeded support to a carbon nanotube precursor gas to grow carbon nanotubes on the surface of the seeded support material.
In another aspect, the invention provides a porous carbon-nanotube-containing structure that includes a large pore support having through porosity; and carbon nanotubes disposed over the large pore support.
In still another aspect, the invention provides a method of making a carbon-nanotube-containing structure in which a surfactant template composition (a composition containing a surfactant and silica or silica precursors) is applied onto a support. Carbon nanotubes are then grown over the layer made from the surfactant template composition.
The invention also provides processes of using carbon nanotube-containing structures. Preferably, any of the carbon nanotube-containing structures described herein can be used in processes including: adsorption, ion exchange, separation of chemical components, filtration, storage of gases (for example, hydrogen or carbon dioxide), distillation (including reactive distillation), as a support structure for chemical or biological sensors, and as a component in a heat exchanger. Features of carbon nanotube-containing structures that make these structures particularly advantageous include: high surface area, excellent thermal conductivity, capillary force for enhanced condensation, and good attractive force for certain organic species.
Thus, the invention provides a method of adsorbing a chemical component in which a chemical component is contacted with a carbon nanotube-containing structure and the chemical component is adsorbed on the surface of the carbon nanotube-containing structure. A preferred chemical species is hydrogen. In a preferred embodiment, the exterior surface of the carbon nanotube-containing structure is a palladium coating. In preferred embodiments, the adsorption is run reversibly in a process such as pressure swing or temperature swing adsorption. This method is not limited to adsorbing a single component but includes simultaneous adsorption of numerous components.
Similarly, the invention provides a method of separating a chemical component from a mixture of components. xe2x80x9cMixturexe2x80x9d also includes solutions, and xe2x80x9cseparatingxe2x80x9d means changing the concentration of at least one component relative to the concentration of at least one other component in the mixture and preferably changes the concentration of at least one component by at least 50% (more preferably at least 95%) relative to at least one other componentxe2x80x94for example reducing the concentration of a 2M feed stream to 1M or less. Particular types of separations include filtration, selective adsorption, distillation and ion exchange. Filtering can be accomplished, for example, by passing a mixture having at least two phases through a porous carbon nanotube-containing structure where at least one of the phases gets physically caught in the structure. A carbon nanotube-containing structure with surface-exposed carbon nanotubes can function efficiently for the separation of some organics because the nanotubes can be hydrophobic while organics can be adsorbed quite well. For ion exchange it is desirable to coat the surface with an ion exchange agent.
The preparation of porous materials, such as foams, coated with carbon nanotubes and a high-surface area metal oxide coating, can be difficult. The locally aligned nanotubes exhibit high surface Van der Waal forces and hydrophobic properties. Conventional wash coating of metal oxides using aqueous based solution often results in a non-uniform coating or poor adhesion onto the nanotubes. We have developed treatment methods to modify the surface properties of the nanotubes, making this new class of materials possible for application as engineered catalyst structure. We have fabricated carbon nanotube-based engineered catalyst and have demonstrated its performance for Fisher-Tropsch reaction in a microchannel reactor. Under operating conditions typical of microchannel reactors with minimal heat and mass transfer limitations, it was found that the integrated nanotubes substrate has further improved the performance, as indicated by enhanced reaction rate and improved product selectivity. This concept can also be applied toward conventional reactors, which operate under severe heat and mass transfer inhibitions with catalyst performance far less than that predicted from the intrinsic kinetics.
Various embodiments of the present invention can offer numerous advantages, including: creating larger pores through which reactants/products transport to the catalytic sites, improved heat transport, controlling the direction of heat transport, enhanced surface area, excellent thermal stability, excellent thermal conductivity, reduced mass transfer limitations, utility in microreactors, ready adaptability in fixed-bed type reactors, and increased catalyst loading levels.
The surface area enhancement that arises from these nanoscale fibers can greatly increase the catalyst site density within a fixed reactor volume. The potential to create larger pore size naturally generated from the interstices between carbon nanotubes can be beneficial for reactions involving both gas and liquid phases liquid reactants or products on a solid catalyst, since the transport of gas phase molecules through the liquid phase inside the pores is often the rate-limiting step which not only hinders the reaction rate but also adversely affects product selectivity.
In this application, xe2x80x9cpore sizexe2x80x9d and xe2x80x9cpore size distributionxe2x80x9d can have different meanings as explained below. xe2x80x9cPore sizexe2x80x9d can be measured by (optical or electron) microscopy where pore size distribution and pore volume are determined statistically from counting in a field of view (of a representative portion of the material) and pore size of each pore is the average pore diameter. Pore size is determined by plotting pore volume (for large pore materials the volume of pores having a size of less than 100 nm can ignored) vs. pore size and xe2x80x9caverage pore sizexe2x80x9d is the pore size at 50% of the existing pore volume (e.g., for a material that has a 40% pore volume, the xe2x80x9caverage pore sizexe2x80x9d is the size of the largest sized pore that adds with all smaller sized pores to reach 20% pore volume). Where practicable, the pore size and pore volume are measured on a cross-section of the material that may be obtained with a diamond bladed saw. For an isotropic material any representative cross-section should produce the same results. For anisotropic materials the cross-section is cut perpendicular to maximum pore length.
Alternatively, pore size and pore size distribution can be measured by nitrogen adsorption and mercury porisimetry.
A xe2x80x9clarge porexe2x80x9d support (or other material) is a support that is characterized by the presence of pores having a pore size (diameter) of at least 100 nm, more preferably at least 1 xcexcm, and in some embodiments 500 nm to 400 xcexcm. Preferably, these supports have through porosity, such as in honeycombs, foams or felts.
xe2x80x9cThrough porosityxe2x80x9d means that (1) when a xe2x80x9cthrough porosityxe2x80x9d material is sized (sized means cut or grownxe2x80x94that is, a through porosity material need not be 1 cm in length, but for testing purposes could be grown or manufactured) to a length of 1 cm (or at least 0.1 cm if 1 cm is unavailable) and oriented in the direction of maximum flow, a measurable amount of argon gas will flow through the intact material, and (2) a cross-section taken at any point perpendicular to flow (for example, where the material is disposed within a reactor) shows the presence of pores, and, in the large pore materials, the presence of large pores. In the present invention, the interstices between packed, unsintered powder particles or pellets do not qualify as through porosity (although powders sintered to form larger materials would qualify). By definition, materials having only pitted surfaces (such as anodized aluminum) do not have through porosity, and mesoporous silica (by itself) does not have through porosity. Anodized aluminum is not a through porosity material.
A xe2x80x9ccarbon nanotubexe2x80x9d is primarily or completely carbon in a substantially cylindrical or rod-like form having a diameter of less than 200 nm, preferably in the range of 4 to 100 nm. xe2x80x9cNanotubesxe2x80x9d may include both tubes and rods.
An xe2x80x9cengineered catalystxe2x80x9d means a catalyst having a porous support, carbon nanotubes, and a catalytically active material disposed over at least a portion of the nanotubes.